Method for Controlling Luminance of Transmissive Board and Transmissive Board

ABSTRACT

A method for controlling luminance of a transmissive board at the moment of transmissive illumination, in a short time, at a low cost. The luminance of a control object is made the target luminance by a rough control step of printing ink having a constant light transmittance on a front side or a back side of the control object in a transmissive board. The rough control step is repeated a number times within a range such that the luminance does not exceed the target luminance. The ink is printed on the front side or the back side by controlling the amount of the ink to be printed to a unit area in accordance with a luminance difference between the target luminance and the control luminance which is not eliminated in the rough control step.

TECHNICAL FIELD

The present invention relates to a method for controlling luminance oftransmissive board and a transmissive board the luminance of which iscontrolled.

BACKGROUND ART

The luminances of a transmissive board such as an instrument board for avehicle sometimes varies depending on the position of a light source orthe distance from the light source. As a method of equalizing theluminance, the following method has been already known (see the patentdocument 1 and the patent document 2). The method includes the step ofphotographing the front side of the transmissive board at the moment oftransmissive illumination, then the image data of the board is convertedto electric signals, the step of implementing the dots disassembly basedon the information of the electric signals to make an original plate forprinting, and the step of providing dots to a transmissive board basedon the original plate for printing.

Patent Document 1: Japanese Patent Laid-Open No. 60122947 PatentDocument 2: Japanese Patent Laid-Open No. 6361117 DISCLOSURE OF THEINVENTION Problem to be Solved by the Invention

However, as a screen printing is applied to the above mentioned method,an original plate for printing and a printing plate are required. Thatcauses a lot of steps and high costs. Moreover, in the screen printing,the minimum dot-diameter of dot depends on the size of an opening ofsilk gauze used for the printing plate. Therefore, for controlling theluminance, such a tangled work, so-called ink toning, is needed, thatthe following steps are repeated several times trough a trial and errorprocess: the step of printing on the board by using inks havingdifferent transmittance from each other, the step of illuminating andphotographing the printed board, and the step of checking the state ofthe luminance. Accordingly, a great deal of time and workload areneeded.

Therefore, the aim of the present invention is to provide a method forcontrolling luminance of a transmissive board at the moment oftransmissive illumination in a short time at a low cost and atransmissive board the luminance of which is controlled by the method.

Means for Solving the Problem

The method of controlling luminance of the present invention is themethod for controlling luminance, in which luminance as control objectin luminance of a transmissive board at a moment of transmissiveillumination is controlled to target luminance, the method has thesteps: a rough control step of printing ink having a constant lighttransmittance on a front side or a back side of a part corresponding toluminance as the control object in the board repeatedly a maximum numbertimes within such a range that the luminances do not exceed the targetluminance, a minor control step of printing the ink on the front side orthe back side of the part by controlling amount of the ink to be printedto a unit area in accordance with a luminance difference between thetarget luminance and the luminance as control object which is noteliminated in the rough control step, and thereby solving the aboveproblems.

According to the present invention, as the ink having a constant lighttransmittance is printed to be laminated on the part corresponding tothe luminance as control object in the rough control step, the lighttransmittance of the part can be increased in accordance with thelaminated times by a predetermined ratio. Therefore, the luminance ofcontrol object can be made closer to the target luminance as much aspossible. For a small range of luminance which can not be controlled byprinting the ink as it is, the luminance can be controlled bycontrolling amount of the ink to be printed to a unit area in the minorcontrol step. Finally, the luminance of control object can be equalizedto the target luminance.

Therefore, the print type of the present invention is enough to controlprint area of ink to a unit area, and requires neither an original platefor printing nor a printing plate. Accordingly, the luminance control ofthe transmissive board can be implemented precisely and quickly. As thestate of printing the ink on the front side or the back side of thetransmissive board, the case where the ink is printed on thetransmissive board directly and the case where the ink is printed on aconstruction mounted on the front side or the back side of thetransmissive board are included. It does not matter which order ofimplementing the rough control step and the minor control step.

In the luminance of the transmissive board at the moment of transmissiveillumination, a lowest luminance may be set as the target luminance, andall of the other luminances may be set as the control object luminance,and the other luminance may be controlled to the lowest luminance. Inthis case, all luminances of the transmissive board at the moment oftransmissive illumination are equalized to the lowest luminance.Thereby, the transmissive board without variant luminance can beobtained.

In the present invention, the amount of ink to be printed to a unit areamay be obtained based on ratio the luminance difference which is noteliminated to a maximum luminance range which can be controlled byprinting the ink one time.

“based on ratio the luminance difference to a maximum luminance rangewhich can be controlled by printing the ink one time” includes, as wellas the case where the ratio is equal to a ratio of amount of the ink tobe printed to the unit area, the case where there is a constantrelationship between the ratio and the ratio of amount of the ink.

The method for controlling to equalize luminance of a transmissive boardis a method for controlling luminance in which a luminance of atransmissive board at a moment of transmissive illumination iscontrolled equally by printing ink having a constant light transmittanceon a front side or a back side of the transmissive board plural times,obtaining a maximum amount of ink by adding an integer and a ratio, theinteger being a integer part of value obtained by dividing luminancedifference between a highest luminance and a lowest luminance at themoment of transmissive illumination by a maximum luminance range whichthe ink can control by one-time print, and the ratio being a ratio ofluminance difference which is not eliminated even if the ink is printedto the maximum luminance range repeated times of the integer, setting alowest luminance as a criterion luminance for each time of print,bringing luminance to the criterion luminance, the luminance beingincluded in a smaller luminance difference than the maximum luminancerange to the criterion luminance, by controlling amount of the ink to beprinted to a unit area in accordance with a ratio of the smallerluminance difference to the maximum luminance range, printing the ink asit is for not smaller luminance difference than the maximum luminancerange, printing the ink repeatedly times of minimum integer over themaximum amount of the ink, and thereby the above problems can be solved.

According to the present invention, first, the maximum ink amount isobtained. The maximum ink amount is the amount of ink which is requiredto eliminate the luminance difference between the highest luminance andthe lowest luminance of the transmissive board. The maximum ink amountcan be obtained by adding the integer part of the value obtained bydividing the luminance difference between the highest luminance and thelowest luminance by the maximum luminance range and the ratio of theluminance difference which can not be controlled by printing the ink tothe maximum luminance range repeatedly times of the integer. The maximumink amount is equal to the amount of ink required to eliminate theluminance difference between the lowest luminance and the highestluminance in the case where the amount of ink required to eliminate theluminance difference of the maximum luminance range is set to 1. Forexample, in the case where the maximum ink amount is 3.2, it means thatthree times of printing the ink as it is and one time of printing theink with the ink amount of 0.2 in the case where a normal amount of inkis set to 1. Therefore, the minimum integer which is equal and more thanthe maximum ink amount is the necessary times of printing the ink

Next, in each turn of printing, for the smaller luminance differencethan the maximum luminance range to the criterion luminance, theluminance corresponding to the smaller luminance difference is made thecriterion luminance by controlling the amount to be printed. On theother hand, for the more luminance difference than the maximum luminancerange, the ink is printed as it is. Thereby, all luminances within themaximum luminance range are made the criterion luminance, that is, thelowest luminance in the printing turn. The luminance in the moreluminance difference than the maximum luminance range reduced by aluminance range corresponding to the maximum luminance range. Thereby,even if there is luminance part which achieves the lowest luminance as afinal target, in another turn of printing the luminance part can be theobject of controlling luminance again. As there is no luminance parthigher than the highest luminance, when the highest luminance is equaledto the lowest luminance, it means that all luminances of thetransmissive board are equalized to the lowest luminance.

The principle of controlling the luminance of transmissive board byprinting the ink, the printing type of the present invention, and theexplanation of “print the ink on the front side and the back side of thetransmissive board” are the similar to the above described contents.Additionally, the above described method for controlling luminance canbe realized as a transmissive board in which luminance has beencontrolled by the method for controlling luminance.

EFFECT OF THE INVENTION

As described above, according to the present invention, by printing tolaminate the ink having a constant light transmittance and printing theink with controlled amount of ink to be printed to a unit area, themethod for controlling in a short time at a low cost the luminance of atransmissive board at the moment of transmissive illumination can beprovided, and the transmissive board controlled the luminance at themoment of transmissive illumination by the method for controllingluminance can be provided.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a view of an example of the transmissive board of the presentinvention.

FIG. 2 is a cross-section diagram of a thermal-transfer ink sheet forcontrolling luminance used in the present embody.

FIG. 3A is a schematic view showing the bright variation in thetransmissive board showed in FIG. 1 at the moment of transmissiveillumination.

FIG. 3B is a view showing a state of shielding light to the area exceptfor the highest luminance area and the lowest luminance area.

FIG. 4A is a view showing image data of the transmissive board showed inFIG. 1 at the moment of transmissive illumination.

FIG. 4B is a view showing a tone curve of the transmissive board at themoment of transmissive illumination.

FIG. 5A is a view showing a state of image data showed in FIG. 4A whichis provided a reversal process and the tone value of which is controlledto range from zero to 256.

FIG. 5B is a view showing a tone curve corresponding to FIG. 5A.

FIG. 6A is a view showing the image data showed in FIG. 5A, in which atone conversion is implemented for a low luminance level.

FIG. 6B is a view showing a tone curve corresponding to FIG. 6A

FIG. 7A is a view showing the image data showed in FIG. 5A, in which atone conversion is implemented for a middle luminance level.

FIG. 7B is a view showing a tone curve corresponding to FIG. 7A.

FIG. 8 is a view showing the image data showed in FIG. 5A, in which atone conversion is implemented for a high luminance level.

FIG. 9 is a view showing a tone curve corresponding to FIG. 8.

FIG. 10A is a view showing an example of division of the luminancelevels of the tone curve showed in FIG. 5B in the case where the maximumnumber of transferred layers is 2.5.

FIG. 10B is a view showing the result of implementing the toneconversion for a high luminance level showed in FIG. 10A.

FIG. 10C is a view showing a different example of division of theluminance levels from the example in 10A.

BEST MODE FOR CARRYING OUT THE INVENTION

FIG. 1 is a view showing an example of a transmissive board forrealizing the present invention. As showed in FIG. 1, the transmissiveboard of the present embodiment is a speed meter 1 for a vehicle. Thespeed meter 1 has a display area 1 c including a numeric area 1 aindicating speed and a level-bar area 1 b. As the numeric area 1 a andthe level-bar area 1 b should be illuminated, shading typed basematerial (no figures) where shading typed ink is printed on the backside of the display area 1 c to shade area except for the numeric area 1a and the level-bar area 1 b. Thereby, when the speed meter 1 isilluminated on the back side thereof by a plural of point light sources(no figures), the area except for the area shaded by the shading typedbase material is transmissively illuminated. As the result of that, thenumeric area 1 a and level-bar area 1 b look like emitting.

In all of the areas 1 a, 1 b to be transmissively illuminated, theposition relationship with the plurality of point light sources is notequal. Therefore, if the transmittance is equal between areas, theluminance of each area is different from each other. The presentinvention is used for solving the luminance variance like this. In thepresent embodiment, the lowest luminance is the target luminance andother luminances are the luminance as control object. In the presentembodiment, a thermal transferring method is applied as a printingmethod, and a thermal-transfer ink sheet 2 for controlling luminanceshowed in FIG. 2 is engaged for printing ink. The thermal-transfer inksheet 2 for controlling luminance showed in FIG. 2 has a base materiallayer 2 a and an ink layer 2 b. The ink layer 2 b has a predeterminedtransmittance to decrease light volume of transmitted light, and istransferred to a portion of luminance as control object on the shadingtyped base material.

When the ink layer 2 b is transferred to the shading typed basematerial, the light volume of light transmitting through the transferredportion decreases according to the transmittance. Accordingly, as theresult of that, the luminance of the transferred portion is controlledlower. As a rough control step, when the ink layer 2 b is transferred asit is without controlling ink volume later described, the luminance iscontrolled always by a same luminance width for each transfer. Theluminance width to be controlled is constant for one transfer, becausethe transmittance of the ink layer 2 b is constant. In the rough controlstep, the luminance width to be controlled is the highest luminancerange which can be controlled by transferring the ink layer 2 b.Hereinafter the luminance width to be controlled is referred as “thehighest control width”. Additionally, if the ink is transferred pluraltimes, the luminance width to be controlled increases in accordance withthe number of times.

As a minor control step, the luminance width smaller than the highestcontrol width is controlled by controlling the volume of the ink layer 2b which is transferred per unit area. In the case where the volume ofink layer 2 b is 1 at the moment of when the ink layer 2 b istransferred to the unit area as it is, the volume of ink layer 2 b to betransferred to the unit area is called an area-ratio. In the presentembodiment, in order to transfer the ink layer 2 b according to thearea-ratio, the transfer volume of ink layer 2 b is controlled bycontrolling the number of dots of the ink layer 2 b to be transferred.Hereinafter, the luminance width to be controlled by the minor controlstep is referred as “the minor control width”. It is referred as only“transfer” that the ink layer 2 b is transferred as it is to the shadingtyped base material by a thermal-transfer printer, and it is referred as“minor-control transfer” that the ink volume of ink layer 2 b istransferred according to the area-ratio.

First, the criterion number of transferred layers in order to controlthe luminance of the highest luminance area at the moment when the speedmeter is transmissively illuminated to the luminance of the lowestluminance area is obtained. The criterion number of transferred layersis the number of transferring the ink layer 2 b which is required forthe highest luminance to be made closest to the lowest luminance so thatthe highest luminance is not made lower than the lowest luminance. Thespeed meter is illuminated on the same conditions as the conditions ofpoint light sources actually used and the highest luminance and thelowest luminance are checked. FIG. 3A is a schematic view showingdifferences between the areas on the speed meter 1 by being illuminated.As the density of stripe is lower, the luminance of the area is higher.Accordingly, in FIG. 3A, the highest luminance area is the area A, andthe lowest luminance area is the area F. Hereinafter, the area A issometimes referred as the highest luminance area A, and the area F issometimes referred as the lowest luminance area F.

In order to shade the areas B-E without the highest luminance area A andthe lowest luminance area F, the shading-typed thermal transfer sheet(for example, black ink) is printed to portions corresponding to theareas B-E on the shading-typed base material. Thereby, the areas B-Ewithout the highest luminance area A and the lowest luminance area F areshaded. FIG. 3B shows the state that the areas B-E are shaded. Under thestate showed in FIG. 3B, the ink layer 2 b of the thermal transfer inksheet for controlling luminance 2 is transferred to a portioncorresponding to the highest luminance area A on the shading-typed basematerial repeatedly from 1 to 5 times and the luminance of the highestluminance area A is checked for each time. The number of transfersshould be specified, at which the luminance of the highest luminancearea A is made closest to the lowest luminance area F and made to behigher than the luminance of the lowest luminance area F. The specifiednumber of transfers is set as the criterion number of transferredlayers.

Next, how to obtain the criterion area-ratio will be described. Thecriterion area-ratio is the area-ratio used when the minor control widthwhich can not be controlled by the above transfer is controlled by theminor-control transfer in order to make the luminance of the highestluminance area A to the luminance of the lowest luminance area F. First,in the similar the way to obtain the criterion number of transferredlayers, the area B-E except for the highest luminance area A and thelowest luminance area F are shaded and the ink layer 2 b is transferredto the portion corresponding to the highest luminance area A up to thecriterion number of transferred layers. After that, for each of caseswhere the area-ratio of transferred ink layer 2 b is 0%, 20%, 40%, 60%,80% and 100%, the ink layer 2 b is minor-control transferred to theportion corresponding to the highest luminance area A, and for eachcase, the luminance of the highest luminance area A after theminor-control transfer is checked. The area-ratio 0% means the casewhere no ink layer 2 b is transferred, and the area-ratio 100% means thecase where the ink layer 2 b is transferred as it is.

The area-ratio which makes the luminance of the highest luminance area Aclosest to the luminance of the lowest luminance area F is checked, andthe area-ratio is set as the criterion area-ratio. In the case of thecriterion area-ratio, it is no matter whether the luminance of thehighest luminance area A is higher or lower than the luminance of thelowest luminance area F. The sum of the criterion number of transferredlayers and the criterion area-ratio obtained by the above describedmethod is the maximum ink volume, and hereinafter referred as “themaximum number of transferred layers”. For example, the maximum numberof transferred layers in the case where the criterion number oftransferred layers is 3 and the criterion area-ratio is 30%, the maximumnumber of transferred layers is 3+0.3=3.3. In this embodiment, themethod for equalizing the luminances of the speed meter 1 in the casewhere the maximum number of transferred layers is 3 will be described.

After the maximum number of transferred layers are obtained, the frontof the speed meter 1 at the moment of transmissive illumination isphotographed by a digital camera to obtain a luminance distributionimage data (hereinafter referred as “the image data” only) which is thedigital data of photographed image. The speed meter 1 should bephotographed under the conditions where the luminance of the highestluminance area A is not higher than the highest luminance of the digitalprocess so that the photographed image is obtained without white effect.For example, if the photographed image is treated as a 256-colorgrayscale image data, the tone value of the highest luminance area A ofthe photographed image data should be set to 255.

FIG. 4A and FIG. 4B show the state of the image data and the tone curveof the image data in the case where the image photographed under theabove conditions is the 256-color grayscale image data. The tone valueof each of areas A-F is obtained from the image data showed in FIG. 2Aby implementing digital processes to each of areas A-F. In each of areasA-F, as the stripe density is higher, the luminance is lower, that is,the tone value is lower. In the present embodiment, the tone value ofarea A is 245, the tone value of area B and area C is 200, the tonevalue of area D is 120, the tone value of area E is 70, and the tonevalue of area E is 40. Although more kinds of tone value are distributedin actual image data, the luminance control only for areas A-F showed inFIG. 4A will be described for convenience sake. The lateral axis of thetone curve showed in FIG. 4B indicates the luminance of the speed meter1 at the moment of transmissive illumination, the left end indicates“the lowest luminance” and the right end indicates “the highestluminance”. The vertical axis indicates the tone value of image data ofthe photographed speed meter 1. The bottom indicates zero and the topindicates 256. The tone value corresponding to the lowest luminance is40 which is the tone value of the lowest luminance area F, and the tonevalue corresponding to the highest luminance is 245 which is the tonevalue of the highest luminance area A.

Next, the tone curve showed in FIG. 4B is controlled so that the tonevalue of the highest luminance area A is set to the maximum tone value256 and the tone value of the lowest luminance area F is set to the tonevalue zero in the present invention. The result of the control is showedin FIG. 5A and FIG. 5B. FIG. 5A shows the state of implementing theabove described control to tone-convert the image data showed in FIG.4A. The highest luminance area A is showed as a state representing thetone value 256, the lowest luminance area F is showed as a staterepresenting the tone value zero. As the stripe density of area ishigher, the ink volume printed in the area is more. Much the same istrue on the image data showed in drawings later described. FIG. 5B showsthe tone curve a relating each luminance of the image data showed inFIG. 5A to each tone value. As the tone curve a shows, the tone valueranging from zero to 256 distributes widely at this moment. However, bythe method described later, all of the tone values can be made the tonevalue zero in the tone curve α. Namely, the tone values of all areas canbe equalized to the luminance of the lowest luminance area F.

Hereinafter, the method for equalizing the luminance of areas A-F isequalized to the lowest each other. Hereinafter, the tone valuecorresponding to area A is referred as the tone value A-F, and theluminance corresponding to the areas A-F before being controlled isreferred as the luminance A-F. First, luminance level is divided basedon the maximum number of transferred layers. The number of division isequal to the number of printing thermal transfer ink sheet forcontrolling luminance 2 which is necessary to lower the highestluminance A to the lowest luminance F. As above described, the number ofprintings is the minimum integer not lower than the maximum number oftransferred layers. For example, in the case where the maximum number oftransferred layers is 2.2, the division number is 3, and in the casewhere the maximum number of transferred layers is 2.0, the divisionnumber is 2. AS the maximum number of transferred layers is 3.0 in thepresent embodiment, as showed in FIG. 5B, the luminance level is dividedinto 3 levels, that is, x, y, and z.

The width of the divided luminance levels X-Z will be described. Eachwidth of the luminance level for an integer part corresponds to themaximum control width. Each width of luminance level for a decimal partcorresponds to a minor control width with respect to the area A. Themaximum control width is constant as above described. Accordingly, thewidth of luminance level is equal to each other in the presentembodiment. The width of luminance level for a minor control widthshould be set so that the ratio of the minor control width to themaximum control width is equal to the criterion area-ratio. Namely, thecriterion area-ratio in the present embodiment is equal to the ratio ofthe minor control width to the maximum control width with respect to themaximum luminance.

For example, in the case where the maximum number of transferred layersis 2.2, the division number of luminance level is 3, and the ratio ofthe widths of each of the luminance levels is 1:1:0.2. In thisembodiment, as the maximum number of transferred layers is 3.0, asshowed in FIG. 5B, the width of the luminance levels X-Z is equallydivided into 3. Hereinafter, each of the luminance levels from thehigher luminance level is called the high luminance level X, the middleluminance level Y, and the low luminance level Z. Each of the luminancelevels corresponds to a high tone value area X′, a middle tone valuearea Y′, and a low tone value area Z′ by a tone curve α. Each of theluminance levels X-Z and each of the tone value areas X′-Z′ is themaximum control width in this embodiment. The area which the minimumluminance level of the middle luminance level Y indicates is “Q”, andthe area which the minimum luminance of the high luminance level Xindicates “R”. The tone value corresponding to the each luminance ofareas Q and R is “Q′” and “R′”.

In order to equalize the luminance A-E to the minimum luminance F, alltone values included in the tone value areas X′-Z′ should be made to thetone value zero of the tone curve α. The method for making the tonevalues to the tone value zero by the transfer and the minor-controltransfer of the ink layer 2 b. Each of the tone value areas X′-Z′ is themaximum control width. Accordingly, by transferring the ink layer 2 b,the tone value Q′ one time is made to be the tone value zero, the tonevalue R′ is made to be the tone value Q′, and the tone value 256 is madeto be the tone value R′. Moreover, for example, in the case where thetone value K which is not the border value of each tone value area X′-Z′is made to be the tone value zero, as the maximum control width is fromthe tone value Q′ to the tone value zero, the ink layer 2 b should betransferred one time, and the minor-control transfer should beimplemented in accordance with the area-ratio to make the luminancedifference, which is the minor control width from the tone value K tothe tone value Q′, to the tone value zero. In this embodiment, thearea-ratio for making the minor control width of each tone value zero(herein after referred to as “necessary area-ratio”) is equal to theratio of the minor control width to be controlled to the maximum controlwidth. Such ratio of the width is constant for each tone value.Consequently, the necessary area-ratio is always constant to each tonevalue. Therefore, each tone value can be corresponded to the necessaryarea-ratio in advance. Accordingly, such correspondence relations can beset as a table in advance, and the necessary area-ratio can be obtainedfrom the tone value by referring the table.

In this embodiment, each of the tone value areas x′-Z′ the tone value iscontrolled independently. First, the processing for low tone value areaZ′ will be described. As the result, the low tone value area Z′ isdivided into 256 tones and all of the tone values included in the middleluminance level Y and the high luminance level X are made 256.

Image data, where the low tone value area Z′ is controlled by the tonecurve α, is showed in FIG. 6A and the tone curve β of the result ofcontrol is showed in FIG. 6B. All of the tone values of areas A-Dwithout areas E and F belonging to the low luminance level Z are 256,and the tone value of area E is E′. The tone value zero corresponds tothe luminance of the area F which is the criterion luminance, and Z′ isthe maximum control width Z, the e′ between the tone values from zero toE′ is the minor control with about the luminance E. In the processingabout the low tone value area Z′, the ink layer 2 b should betransferred or minor-control transferred so that all of the tone valuesof areas A-F indicated by the tone curve β are made zero. Accordingly,there is no need to do something about the minimum luminance area F, andabout the areas A-D the tone value of which is 256 the ink layer 2 bshould be transferred by the area-ratio 100%. About area E, thenecessary area-ratio corresponding to the tone value E′ should beobtained from the above described table and the ink layer 2 b should beminor-control transferred with the necessary area-ratio.

Next, the processing for the middle tone value area Y′ will bedescribed. First, the tone curve α in FIG. 5B should be controlled sothat the tone values not over the minimum tone value Q′ in the middletone value area Y′ are brought to zero and the tone values over themaximum tone value R′ are brought to 256. As the result, the middle tonevalue are Y′ are divided into 256 tone values. All of the tone valuescorresponding to the low luminance level Z are brought to zero and allof the tone values corresponding to the high luminance level X arebrought to 256. About the middle tone value area Y′, the image datarepresenting the result of controlling the tone curve α is showed inFIG. 7A. The tone curve y as the result of control is showed in FIG. 7B.The tone value of areas E and F included in the low luminance level Z iszero, the tone value of areas A, B, C included in the high luminancelevel X is 256, and the tone value of area D included in the middleluminance level Y is D′.

The luminance level zero corresponds to the luminance level of area Q asthe criterion luminance, Y′ is the maximum control width, the tonevalues d′ from zero to D′ is the minor control width about the luminanceD. In the processing about the middle tone value area Y′, the ink layer2 b should be transferred or minor-control transferred so that all ofthe tone values of areas A-F indicated by the tone curve y are madezero. Accordingly, in the similar way to the low luminance level Z, theink layer 2 b should be transferred by the transfer area-ratio 100%about areas A-C the tone value of which is 256. About areas E and F thetone value of which is zero, the transfer is not needed. About area D,the necessary area-ratio corresponding to the tone value D′ should beobtained from the above described table and the ink layer 2 b should betransferred with the necessary area-ratio.

Next, the processing for the high tone value area X′ will be described.First, the tone curve a should be controlled so that the maximum tonevalue in the high tone value area X remains to be 256 and all of thetone values not over the minimum tone value R′ are brought to zero. Asthe result, the high tone value area X′ is divided into 256, all of thetone value corresponding to the middle luminance level and the lowluminance level is brought to zero. Image data representing the resultof control the tone curve a about the high tone value area X′ is showedin FIG. 8. The tone curve δ obtained by the result of control is showedin FIG. 9. The tone value of the areas E and F in the low luminancelevel Z and the tone value of the area D in the middle luminance level Yis zero. Each of the tone values of areas B and C in the high luminancelevel X is B′ and C′ respectively.

The tone value zero corresponds to the luminance of area R as thecriterion luminance. X′ is the maximum control width, the tone values b′from zero to B′ is the minor control width about the luminance B. In theprocessing about the high tone value area X′, the ink layer 2 b shouldbe transferred or minor-control transferred so that all of the tonevalues of areas A-F indicated by the tone value α are brought to zero.Accordingly, in the similar way to the low luminance level Z and themiddle luminance level X, about the area A the tone value of which is256, the ink layer 2 b should be transferred by the transfer area-ratio100%, and the transfer of layer 2 b is not needed about areas D-F thetone value of which is zero. About area B, the necessary area-ratiocorresponding to the tone value B′ should be obtained and the ink layer2 b should be minor-control transferred by the necessary area-ratio.Also, the similar processing to the area B should be implemented aboutthe area C.

In the above described processing, the tone values of areas A-E arebrought to the minimum luminance in each of the tone value areas X′-Z′for each of the tone value areas X′-Z′. Accordingly, when processing forall tone value areas X′-Z′ ends, the luminances A-E as the controlobject is brought to the tone value zero corresponding to the minimumluminance F. Namely, all of the luminances A-E can be brought to theminimum luminance F equally. In the case of the present embodiment, bythe above described processing, the ink layer 2 b is transferredrepeatedly tree times about the area A finally, about areas B and C theink layer 2 b is transferred repeatedly two times and minor-controltransferred with the obtained necessary area-ratio one time, about areaD the ink layer 2 b is transferred one time and minor-controltransferred with the obtained necessary area-ratio, and about area E theink layer 2 b is minor-control transferred with the obtained necessaryarea-ratio only. Thereby, the luminance of each area A-E can be equal tobe the luminance of area F.

the present invention is not limited to the above described embodimentand may be realized in various embodiments. For example, the maximumnumber of transferred layers may not be an integer and can have thedecimal part. For example, the case of 2.5 will be described. In thetone curve a showed in FIG. 5B, the luminance levels should be dividedso that the ratio between the width of high luminance level Xa, thewidth of middle luminance level Y, and the width of low luminance levelZ is 0.5:1:1. In the case where the maximum number of transferred layersis 2.5, the tone curve a in the state where the luminance levels hasbeen divided is showed in FIG. 10A. The processing about the middleluminance level Y and the low luminance level Z is similar to the abovedescribed processing about the middle luminance level Y and the lowluminance level Z respectively. Therefore, the description of theprocessing is omitted.

The processing of the high tone value area Xa′ corresponding to the highluminance level Xa will be described. The tone curve α showed in FIG. 5Bshould be controlled so that the maximum tone value in the high tonevalue area Xa′ is made the value “256×criterion area-ratio”, and all ofthe tone values not over than the minimum tone value R′ are made zero.As the result of the control the tone curve ε is showed in FIG. 10B. Thetone value corresponding to the low luminance level Z and the middleluminance level Y is zero, the highest value of tone value in the highluminance level Xa is 128 obtained by 256×0.5. The tone values in thehigh tone value area Xa′ are divided into the tone values from zero to128. In this manner, the maximum tone value of the luminance level Xacorresponding to the decimal part of the maximum number of transferredlayers can be obtained by the maximum control width×the criterionarea-ratio. The part of the curve corresponding to the high luminancelevel Xa of the tone curve ε is the part corresponding to the criterionarea-ratio of the same curve as each of luminance levels Z and Y of theother tone curves β and γ. Accordingly, the necessary are ratiocorresponding to each of the tone values in the high tone value area Xa′can be also obtained by the above described table.

Moreover, in the case where the maximum number of transferred layers hasthe decimal part, it is assumed that the luminance level correspondingto the decimal part is the high luminance level Xa. However, theluminance level corresponding to the decimal part is not needed to bethe high luminance level, and may be any level of luminance. Forexample, in the case where the maximum number of transferred layers is2.5, the middle luminance level Ya may be set as the luminance levelcorresponding to the decimal part, and the ratio between the width ofthe high luminance level X, the width of the middle luminance level Ya,and the width of the low luminance level Z may be set as 1:0.5:1.

Moreover, in the present embodiment, the processing of making the tonevalue of each luminance level zero from the lower luminance level isdescribed. However, it doesn't matter which order of the processing forthe divided luminance levels. For example, the luminance of each tonevalue may be made the criterion luminance from the higher luminancelevel. The criterion luminance in the processing for each luminancelevel is the luminance of the areas F, Q, R corresponding to the minimumluminance of the luminance level X-Z before the processing of luminancecontrol respectively.

In the present embodiment, the method of determining the height ofluminance by eye is described. However, by obtaining the digital imagedata of the front of speed meter 1 at the moment of transmissiveillumination, the height of luminance may be determined based on thedigital image data. Moreover, as above described, the maximum number oftransferred layers is the ink amount required to make the highestluminance the lowest luminance. In the case where such ink amount isdetermined in advance, it is not necessary to implement the processingfor obtaining the maximum number of transferred layers in the presentembodiment. Also, in the case where the maximum number transfer layersis obtained by calculation, such as the case where the maximum controlwidth and the minor control width for the highest luminance are known inadvance, it is not necessary to implement the processing for obtainingthe maximum number of transferred layers in the present embodiment.Moreover, throughout from the processing of obtaining the maximum numberof transferred layers to the processing of transferring the ink layer 2b in accordance with each luminance level, the process can beimplemented by a computer. The process after obtaining the maximumnumber of transferred layers can be implemented by a computer. Thetransfer number in the method for obtaining the criterion number oftransferred layers and each area-ratio in the method for obtaining thecriterion area-ratio are limited to the present embodiment and can bechanged appropriately.

As the method for controlling the amount of ink layer 2 b to be printedto a unit area, the amount is controlled by number of printed dots inthe present embodiment. However, it can be controlled by the size ofprinted dot. Moreover, the criterion area-ratio may be obtained byselecting area-ratios which bring the luminance as the control objectclose to the target luminance and averaging the selected area-ratios.Moreover, the printing method for realizing the present invention is notlimited to a thermal transfer print, as long as the ink having a fixedtransmittance can be printed by one-time print and the ink amount to beprinted to a unit area can be controlled. For example, ink-jet printingand dot-impact printing can be applied.

In the present embodiment, the method is described that all luminancesbefore luminance control processing are equalized to the lowestluminance as the target luminance. However, different target luminancecan be set depending on luminance or position of the processing object.In this case, by different luminance, a predetermined shape or colorgradation can be obtained. Moreover, color tone can be controlled bydividing the image data photographed by a digital camera into light'sthree primary colors, and controlling the luminance corresponding toeach color by the luminance control method of the present invention.

Moreover, the maximum tone value is not limited to 256 and can bechanged appropriately depending on the type of print system or purpose.The transmissive board is not limited to a speed meter 1 and can be anykind of a board as long as the board is illuminated from the backthereof such as various kinds of boards of instruments. Moreover, as themethod for obtaining the area-ratio to bring each tone value to zero, inthe present embodiment, the method is employed that the table in whichthe tone value and the area-ratio are corresponded to each other isprepared in advance. However, in the case where there is a constantrelationship between the tone value and the area-ratio, the area-ratiocan be obtained, for example, from an equation having a variable as thetone value for obtaining the relationship.

1. The method for controlling luminance, in which luminance as controlobject in luminance of a transmissive board at a moment of transmissiveillumination is controlled to target luminance, the method has thesteps: a rough control step of printing ink having a constant lighttransmittance on a front side or a back side of a part corresponding toluminance as the control object in the board repeatedly a maximum numberof times within such a range that the luminances do not exceed thetarget luminance, a minor control step of printing the ink on the frontside or the back side of the part by controlling amount of the ink to beprinted to a unit area in accordance with a luminance difference betweenthe target luminance and the luminance as control object which is noteliminated in the rough control step.
 2. The method for controllingluminance according to claim 1, wherein in the luminance of thetransmissive board at the moment of transmissive illumination, a lowestluminance is set as the target luminance, and all of the otherluminances are set as the luminance as control object, and the otherluminances are controlled to the lowest luminance.
 3. The method forcontrolling luminance according to claim 1, wherein the amount of ink tobe printed to a unit area is obtained based on ratio the luminancedifference which is not eliminated to a maximum luminance range whichcan be controlled by printing the ink one time.
 4. A method forcontrolling luminance in which a luminance of a transmissive board at amoment of transmissive illumination is controlled equally by printingink having a constant light transmittance on a front side or a back sideof the transmissive board plural times, obtaining a maximum amount ofink by adding an integer and a ratio, the integer being a integer partof value obtained by dividing luminance difference between a highestluminance and a lowest luminance at the moment of transmissiveillumination by a maximum luminance range which the ink can control byone-time print, and the ratio being a ratio of luminance differencewhich is not eliminated even if the ink is printed to the maximumluminance range repeatedly times of the integer, setting a lowestluminance as a criterion luminance for each time of print, bringingluminance to the criterion luminance, the luminance being included in asmaller luminance difference than the maximum luminance range to thecriterion luminance, by controlling amount of the ink to be printed to aunit area in accordance with a ratio of the smaller luminance differenceto the maximum luminance range, printing the ink as it is for notsmaller luminance difference than the maximum luminance range, printingthe ink repeatedly times of minimum integer over the maximum amount ofthe ink.
 5. A transmissive board in which luminance has been controlledby the method for controlling luminance according to claim
 1. 6. Themethod for controlling luminance according to claim 2, wherein theamount of ink to be printed to a unit area is obtained based on ratiothe luminance difference which is not eliminated to a maximum luminancerange which can be controlled by printing the ink one time.
 7. Atransmissive board in which luminance has been controlled by the methodfor controlling luminance according to claim 4.